1. Field of the Invention
The present invention relates to a variable resistor, and more particularly, it relates to an open type variable resistor.
2. Description of the Background Art
Surface-mountable miniature pre-set variable resistors can be classified into open type ones such as that described in Japanese Utility Model Laying-Open No. 63-105303 (1988), for example, and casing type ones such as that described in Japanese Utility Model Publication No. 3-9286 (1991), for example.
A casing type variable resistor is highly reliable in maintenance of characteristics, since substantially no flux penetrates into its interior in a soldering step and no inconvenience results from such penetration. However, a variable resistor of this type has a relatively large number of components, and hence the cost therefor is increased. Therefore, an open type variable resistor, which is superior in economy to the casing type one, is mainly employed in recent years.
FIG. 4 illustrates an open type variable resistor 1, which is described in the aforementioned Japanese Utility Model Laying-Open No. 63-105303 (1988). This variable resistor 1 comprises a substrate 2 of an electrical insulating material, which is provided with a through hole 3. A horseshoe resistor film 4 is formed on an upper major surface of the substrate 2. A fixed side terminal 5, which is electrically connected to the resistor film 4, is located along one side surface of the substrate 2. A slider terminal 6 is located along a lower major surface and the other side surface of the substrate 2. This slider terminal 6 comprises a cylindrical portion 7, which is positioned in the through hole 3. This cylindrical portion 7 is provided on its upper end with a crimped portion 8, so that a slider 9 is held between the crimped portion 8 and the substrate 2 to be rotatable with respect to the substrate 2. The slider 9 has a contactor 10, which slides along the resistor film 4 upon rotation of the slider 9.
Thus, an electrical path is defined between the fixed side terminal 5 and the slider terminal 6 through the resistor film 4 and the slider 9, and resistance across the terminals 5 and 6 is varied with rotation of the slider 9.
This variable resistor 1 is intended to be lifted by vacuum suction when the same is mounted on a proper circuit board. Therefore, a folded portion 11 is formed integrally with the slider terminal 6, thereby blocking the through hole 3. Thus, the through hole 3 is prevented from causing air leakage during vacuum suction.
However, the aforementioned variable resistor 1 may encounter the following problem: When this variable resistor 1 is soldered onto a proper circuit board by reflow soldering utilizing creamed solder, for example, flux which is contained in the creamed solder may reach the upper major surface of the substrate 2 through surfaces of or clearances between specific elements forming the variable resistor 1, as shown by arrows 12 and 13 in FIG. 4. The flux consequently adheres to the resistor film 4, the slider 9 and the like, to deteriorate the characteristics of the variable resistor 1.
In order to solve the aforementioned problem, it is possible to wash out the flux adhering to undesired portions with an organic solvent. In this case, however, a washing step is additionally required and hence the manufacturing cost for a circuit board mounted with the variable resistor 1 is disadvantageously increased. Further, such washing with an organic solvent is now being restricted in view of environmental protection, and hence it is rather unpreferable to depend on such washing with an organic solvent.